In recent years, lightweight design has helped achieve more energy efficient vehicles. It has also been estimated that fuel economy may increase 6 to 8% if vehicle weight is decreased by 10%. Lightweight design, however, may increase structural vibration and consequently interior noise, especially at low frequencies. And, passive noise control may not be ideal because it tends to add to vehicle weight and cost. As such, active noise control (ANC) technology has been developed that uses the audio system as a secondary speaker to control engine noise, powertrain noise and road noise.